Wire rod

ABSTRACT

A wire rod according to an aspect of the present invention includes a chemical composition within a predetermined range; in which an average value of % Mn+2 x % Cr over an entirety of the wire rod is 0.50% to 1.00%; 90% or more of a metallographic structure is pearlite by area fraction, and the area fraction of the cementite is less than 3%; a maximum grain size of TiN is less than 15 μm; a maximum value of % Mn+2 x % Cr in a region where both a S content and an 0 content are less than 1% in a central portion is 2.0 times or less than the average value of % Mn+2x % Cr over the entirety of the wire rod, and a ratio of the maximum value to a minimum value of % Mn+2x % Cr in a region where both a S content and an 0 content are less than 1% in an outer circumferential portion is 2.0 or less.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a wire rod. Priority is claimed on Japanese Patent Application No. 2016-246866, filed on Dec. 20, 2016, the content of which is incorporated herein by reference.

RELATED ART

A drawn steel wire for steel cords used as radial tires of automobiles, various kinds of belts, and reinforcing materials for hoses, or a drawn steel wire for sawing wires is generally obtained by the procedure described below. First, a steel wire rod (hereinafter, a “steel wire rod” will be simply referred to as a “wire rod”) which has a wire diameter (diameter) within a range of 4 to 6 mm and has been control-cooled after hot rolling is subjected to primary wire-drawing to have a diameter within a range of 3 to 4 mm Subsequently, intermediate patenting treatment is performed. Moreover, secondary wire-drawing is performed to have a diameter within a range of 1 to 2 mm. In many cases, the diameter within a range of 1 to 2 mm is realized without performing the intermediate patenting treatment for the sake of cost reduction. Thereafter, final patenting treatment is performed for the drawn steel wire to have the diameter within a range of 1 to 2 mm. Subsequently, brass-plating is performed, and final wet wire-drawing is performed to have a diameter within a range of 0.06 to 0.40 mm. A plurality of high strength drawn steel wires manufactured in this manner to have a small diameter (extra fine drawn steel wire) are twisted through twisting, for example, as a “twisted drawn steel wire”, thereby becoming a steel cord or the like.

Generally, if wire breaking occurs when a wire rod is made into a drawn steel wire or when a drawn steel wire is subjected to twisting, the productivity and the yield are significantly deteriorated. Therefore, wire rods which belong to the foregoing technical field or drawn steel wires made of the wire rods are strongly required not to cause wire breaking at the time of wire-drawing or at the time of twisting. Particularly, in the final wet wire-drawing of wire-drawing, the wire diameter of a workpiece becomes narrow compared to that in primary wire-drawing and secondary wire-drawing, so that the process becomes sensitive to a defect in a wire rod (material), and the drawing length per unit mass increases. Therefore, the frequency of occurrence of wire breaking is high in final wet wire-drawing.

Recently, there is a growing trend towards weight reduction in a steel cord due to various purposes. Therefore, the foregoing drawn steel wires are required to have high strength, and desired strength is achieved by techniques of adding an alloying element, increasing the quantity of final wet wire-drawing, and the like. However, in a case of using the high-strengthening technique described above, the frequency of occurrence of wire breaking is likely to be high in final wet wire-drawing. Accordingly, not much progress has been made in mass production of steel cords having higher strength. Therefore, there is an extremely increasing demand for wire rods having excellent drawability and high strength such that wire breaking in final wet wire-drawing can be prevented.

In response to the foregoing request from the industrial world in recent years, a technology of enhancing the drawability of wire rods and drawn steel wires by using techniques of reducing impurity elements, controlling inclusions, suppressing generation of proeutectoid cementite, managing conditions for hot rolling, and the like has been proposed.

For example, Patent Document 1 discloses a “high-carbon steel wire rod having excellent drawability and twistability”, including, by mass %, C: 0.6% to 1.1%, Si: 0.1% to 1.5%, Mn: 0.2% to 1%, P: 0.025% or less, S: 0.025% or less, and Al: 0.003% or less. As necessary, the steel wire rod further includes Ni, Co, Cu, Cr, and V. The total amount of oxygen, the average composition of non-metal inclusions, and the Ti content are stipulated. In this technology proposed in Patent Document 1, the Ti content is limited to control oxide-based non-metal inclusions, but TiN is not considered. In addition, in the technology proposed in Patent Document 1, segregation of an element is also not considered. Therefore, according to the technology proposed in Patent Document 1, in a case where a working amount, that is, a true strain amount in final wet wire-drawing is increased to achieve high-strengthening, the frequency of occurrence of wire breaking is likely to be high, so that it is difficult to manufacture a drawn steel wire in an industrially stable manner.

Patent Document 2 discloses a “high-carbon steel wire rod having excellent cuppy wire breaking resistance” including, by mass %, C: 0.70% to 0.90%, Si: 0.05% to 1.20%, Mn: 0.10% to 1.0%, Al: 0.05% or less (not including 0%), and a remainder consisting of Fe and unavoidable impurities. The concentration of Si in a cross section of the wire rod is stipulated. In this technology proposed in Patent Document 2, segregation of Mn and Cr is not considered, and TiN is also not considered. Therefore, according to the technology proposed in Patent Document 2, in a case where a working amount, that is, a true strain amount in final wet wire-drawing is increased to achieve high-strengthening, the frequency of occurrence of wire breaking is likely to be high, so that it has been difficult to manufacture a drawn steel wire in an industrially stable manner.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H6-330239

[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2007-297674

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in consideration of the foregoing circumstances, and an object thereof is to provide a wire rod with which a drawn steel wire having excellent drawability and high tensile strength and suitable for the purpose of working of a steel cord, a sawing wire, or the like can be manufactured.

Means for Solving the Problem

In order to solve the foregoing problems, the inventors have first repeated investigations and studies in regard to the influence of chemical compositions, microstructures, inclusions of the wire rod on wire breaking at the time of wire-drawing and could achieve the following knowledge by analyzing and examining the results in detail.

(a) In order to improve the strength of the drawn steel wire after final wire-drawing, it is effective to increase the amount of C and to add Cr.

(b) In order to suppress wire breaking of the wire rod during primary wire-drawing, it is preferable that pearlite is adopted as a main constituent of the structure of the wire rod, the remainder consists of any of ferrite, cementite, and bainite, and no martensite structure is included.

(c) Mn and Cr are elements which are likely to be segregated in the wire rod. Particularly, Mn and Cr are likely to be positively segregated (concentrated) in the central portion of the wire rod. Therefore, strength is easily increased in the central portion of the wire rod, and deformability is thereby deteriorated. Moreover, high tensile stress is applied to the central portion of the wire rod during wire-drawing. Based on this fact, if positive segregation of Mn and Cr in the central portion of the wire rod is significant, wire breaking is likely to occur during wire-drawing. In addition, if Mn and Cr are positively segregated in the central portion of the wire rod, martensitic transformation is likely to occur in the central portion. This fact also becomes a cause for promoting occurrence of wire breaking. Thus, it is necessary that the amount of Mn and the amount of Cr are appropriately controlled. The degree of the influence of Cr on the tensile strength of the wire rod and generation of martensite is approximately twice the influence of Mn.

(d) Mn and Cr are likely to be segregated in a place other than the central portion of the wire rod. In the wire rod, Mn and Cr are likely to be segregated in a band shape. This band-shaped segregation can be observed in a cross section parallel to a rolling direction. The segregated part becomes hard, so that cracking easily proceeds along the segregated part due to wire-drawing. Therefore, band-shaped segregation is likely to cause wire breaking. In this case as well, the degree of the influence of Cr on wire breaking is approximately twice the influence of Mn.

(e) The segregation in the wire rod disclosed in (c) and (d) remains even if patenting treatment is performed after primary wire-drawing, thereby causing the influence described above on drawability up to final wire-drawing.

The inventors have repeated experiments and studies more specifically based on the knowledge of (a) to (e). As a result, it has been found that each of the composition of the wire rod, the condition of metallographic structures having pearlite as a main constituent, the size of TiN, and segregation of Mn and Cr need only be appropriately adjusted. Then, the inventors have checked that the foregoing problems can be solved with the wire rod in which each of these items is within an appropriate range, and a high strength drawn steel wire suitable for a material of a steel cord can be stably manufactured with less frequent wire breaking. Thus, the present invention has been conceived.

The gist of the present invention is as follows.

(1) According to an aspect of the present invention, a wire rod includes, as a chemical composition, by mass %, C: 0.90% to 1.20%; Si: 0.10% to 1.00%; Mn: 0.20% to 0.80%; Cr: 0.10% to 0.40%; Al: 0% to 0.002%; Ti: 0% to 0.002%; N: 0% to 0.0050%; P: 0% to 0.020%; S: 0% to 0.010%; 0: 0% to 0.0040%; Mo: 0% to 0.20%; B: 0% to 0.0030%; and a remainder including Fe and impurities; in which an average value of % Mn+2x % Cr over an entirety of the wire rod is 0.50% to 1.00%; 90% or more of a metallographic structure is pearlite by area fraction, and a remainder of the metallographic structure includes one or more of ferrite, cementite, and bainite, and the area fraction of the cementite is less than 3%; a maximum grain size of TiN is less than 15 μm; a maximum value of % Mn+2x % Cr measured on a cutting surface having a right angle to a longitudinal direction of the wire rod in a region where both a S content and an O content are less than 1% in a central portion, which is a region from a central axis of the wire rod to 1/10 of a diameter of the wire rod, is 2.0 times or less than the average value of % Mn+2x % Cr over the entirety of the wire rod, a ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value to a minimum value in a region where both a S content and an O content are less than 1% in an outer circumferential portion, which is a region from an outer edge of the central portion to a depth of 0.1 mm from a surface of the wire rod, is 2.0 or less, and % Mn and % Cr indicate the amounts of Mn and Cr by mass %.

(2) The wire rod according to (1) may include, as the chemical composition, by mass %, one or more of Mo: 0.02% to 0.20% and B: 0.0003% to 0.0030%.

EFFECTS OF THE INVENTION

According to the wire rod of the present invention, drawn steel wires for steel cords or drawn steel wires for sawing wires and the like having high strength, which for example, tensile strength is 4,100 MPa or more, can be stably manufactured with less frequent wire breaking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a method of measuring concentrations of Mn and Cr in a central portion.

FIG. 2 is a view showing a method of measuring concentrations of Mn and Cr in an outer circumferential portion.

EMBODIMENT OF THE INVENTION

Conditions or the like for the chemical composition and metallographic structures in the wire rod according to an aspect of the present invention (hereinafter, which may be simply referred to as a “wire rod”) will be described in more detail. A wire rod 1 according to the present embodiment includes a central portion 11 which is a region from a central axis of the wire rod to 1/10 of the diameter of the wire rod, and an outer circumferential portion 12 which is a region from an outer edge of the central portion 11 to the depth of 0.1 mm from the surface of the wire rod 1.

<Composition>

Hereinafter, the chemical composition of the wire rod according to the present embodiment will be described. The chemical composition of the wire rod according to the present embodiment described below is based on the amount of alloying elements included in the material of the wire rod, thereby indicating the average value of the chemical composition over the entirety of the wire rod. The chemical composition of the wire rod is measured by analysis measuring the average value of a region having a certain size, instead of local analysis affected by segregation of elements.

C: 0.90% to 1.20%

C is a composition effective in enhancing the tensile strength of the steel. However, in a case where the C content is less than 0.90%, it is difficult to stably apply high strength, for example, such as 4,100 MPa, to a final product. Moreover, in order to stably obtain the final product having high strength, it is effective to increase the C content. In order to obtain the final product having tensile strength of 4,300 MPa or more, it is desirable that the lower limit for the C content is set to 1.00% or more. On the other hand, if the C content is excessive, the steel is hardened, thereby resulting in wire breaking at the time of wire-drawing. Particularly, if the C content exceeds 1.20%, the influence thereof becomes remarkable, so that it is difficult to realize mass production of the drawn steel wire in an industrially stable manner. Therefore, the C content is set within a range of 0.90% to 1.20%. The lower limit value for the C content may be set to 0.92%, 0.95%, or 1.00%. In addition, the upper limit value for the C content may be set to 1.15%, 1.12%, or 1.10%.

Si: 0.10% to 1.00%

Si is also a composition effective in enhancing the strength of the steel and is a composition required as a deoxidizing agent as well. However, if the Si content is less than 0.10%, the effect of Si cannot be sufficiently obtained. On the other hand, if exceeding 1.00% of Si is contained, ductility of the wire rod or the drawn steel wire after wire-drawing is deteriorated. Therefore, the Si content is set within a range of 0.10% to 1.00%. Since Si is an element affecting hardenability of the steel or generation of proeutectoid cementite, from the viewpoint of stably ensuring desired microstructures in the wire rod, it is more desirable that the Si content is adjusted within a range of 0.30% to 0.80%. The lower limit value for the Si content may be set to 0.15%, 0.20%, 0.30%, or 0.40%. In addition, the upper limit value for the Si content may be set to 0.95%, 0.90%, 0.85%, 0.80%, 0.70%, or 0.50%.

Mn: 0.20% to 0.80%

Mn is a composition which affects the time required for phase transformation from austenite to various low-temperature structures and is effective in generating stable pearlite structures in the wire rod. At the same time, Mn has an effect of enhancing the strength of the final product. However, if the Mn content is less than 0.20%, the foregoing effects cannot be sufficiently obtained. On the other hand, since Mn is an element which is likely to be segregated, if exceeding 0.80% of Mn is contained, Mn is segregated particularly in the central portion 11 of the wire rod, so that martensite is generated in the segregated portion and wire breaking occurs at the time of wire-drawing. Therefore, the Mn content is set within a range of 0.20% to 0.80%. The lower limit value for the Mn content may be set to 0.25%, 0.30%, or 0.40%. In addition, the upper limit value for the Mn content may be set to 0.75%, 0.70%, or 0.60%.

Cr: 0.10% to 0.40%

Cr significantly affects the time required for phase transformation from austenite to various low-temperature structures and is effective in generating stable pearlite structures in the wire rod. At the same time, Cr has an effect of enhancing the strength of a final product by reducing lamellar spacing of pearlite. In order to stably achieve tensile strength of 4,100 MPa or more in a final product, it is necessary that the Cr content is 0.10% or more. However, if exceeding 0.40% of Cr is contained, Cr is segregated particularly in the central portion 11 of the wire rod, so that martensite is generated in the segregated portion and wire breaking occurs at the time of wire-drawing. Therefore, the Cr content is set within a range of 0.10% to 0.40%. The lower limit value for the Cr content may be set to 0.12%, 0.15%, or 0.20%. In addition, the upper limit value for the Cr content may be set to 0.35%, 0.30%, or 0.25%.

Average value of % Mn+2x % Cr over entirety of wire rod: 0.50% to 1.00%

As described above, Mn and Cr significantly affect the time for phase transformation from austenite to various low-temperature structures. Accordingly, in order to obtain stable pearlite structures in the wire rod, each of the Mn content and the Cr content has to be set within a predetermined range. However, in addition to the limitation described above, the inventors have ascertained that the total value of the average value of the Mn content over the entirety of the wire rod and the value obtained by multiplying the average value of the Cr content over the entirety of the wire rod by 2 is also required to be set within a predetermined range. Hereinafter, the Mn content will be indicated by the symbol “% Mn”, and the Cr content will be indicated by the symbol “% Cr”.

The effect of Cr affecting tensile strength is approximately twice the effect of Mn. If the average value of % Mn+2x % Cr (hereinafter, which may be disclosed as Mn+2Cr) over the entirety of the wire rod falls below 0.50%, tensile strength of 4,100 MPa or more cannot be stably achieved in a final product. Therefore, the lower limit for the average value of Mn+2Cr over the entirety of the wire rod is set to 0.50% or more. In order to further improve the tensile strength, it is preferable that the average value of Mn+2Cr over the entirety of the wire rod is set to 0.60% or more, 0.70% or more, or 0.80% or more. On the other hand, as described above, since both Mn and Cr are likely to be segregated particularly in the central portion 11 of the wire rod, if the average value of Mn+2Cr over the entirety of the wire rod exceeds 1.00%, martensite is generated in the segregated portion of Mn and Cr, and wire breaking occurs at the time of wire-drawing. Therefore, the average value of Mn+2Cr over the entirety of the wire rod is set within a range of 0.50% to 1.00%. The upper limit for the average value of Mn+2Cr over the entirety of the wire rod may be set to 0.95%, 0.90%, or 0.80%.

The wire rod according to the present embodiment includes the essential elements described above and may further contain the optional elements described below. However, the wire rod according to the present embodiment can solve the problems without including the optional elements. Therefore, the lower limit value for each of the optional elements is 0%.

Al: 0% to 0.002%

Al is an element which forms oxide-based inclusions having Al₂O₃ as a main composition and deteriorates drawability of the wire rod. Particularly, if the Al content exceeds 0.002%, oxide-based inclusions are coarsened and wire breaking frequently occurs during wire-drawing, so that drawability is significantly deteriorated. Therefore, the Al content is restricted to 0.002% or less. Preferably, the Al content is 0.001% or less. As described above, the wire rod according to the present embodiment does not require Al at all in order to solve this problem. Therefore, even if the Al content is 0%, favorable properties can be achieved. Therefore, the lower limit value for the Al content is 0%. On the other hand, there are cases where the manufacturing cost rises due to reduction of the Al content. Thus, it may be stipulated that the Al content is 0.0005% or more, or 0.001% or more.

Ti: 0% to 0.002%

Ti is likely to form TiN if the wire rod contains N. Since TiN is extremely hard and is not deformed due to hot rolling or wire-drawing, TiN is likely to become an origin of wire breaking during wire-drawing. In consideration of a manufacturing method, if the Ti content exceeds 0.002%, it is difficult that the maximum grain size of TiN in the wire rod, which is measured by the method described below, is less than 15 μm, so that wire breaking is likely to occur during wire-drawing. Therefore, the Ti content is restricted to 0.002% or less. Preferably, the Ti content is 0.001% or less. As described above, the wire rod according to the present embodiment does not require Ti at all in order to solve this problem. Therefore, even if the Ti content is 0%, favorable properties can be achieved. Therefore, the lower limit value for the Ti content is 0%. On the other hand, there are cases where the manufacturing cost rises due to reduction of the Ti content. Thus, it may be stipulated that the Ti content is 0.0005% or more, or 0.001% or more.

N: 0% to 0.0050%

N is likely to form TiN if the wire rod contains Ti. Since TiN is extremely hard and is not deformed due to hot rolling or wire-drawing, N is likely to become an origin of wire breaking during wire-drawing. In consideration of the manufacturing method, if the N content exceeds 0.0050%, it is difficult that the maximum grain size of TiN in the wire rod, which is measured by the method described below, is less than 15 μm, so that wire breaking is likely to occur during wire-drawing. Therefore, the N content is restricted to 0.0050% or less. Preferably, the N content is 0.0040% or less.

P: 0% to 0.020%

P is an element which is segregated in grain boundaries and deteriorates drawability. Particularly, if the P content exceeds 0.020%, drawability is significantly deteriorated. Therefore, the P content is restricted to 0.020% or less. Preferably, the P content is 0.010% or less.

S: 0% to 0.010%

S is an element which deteriorates drawability. If the S content particularly exceeds 0.010%, drawability is significantly deteriorated. Therefore, the S content is restricted to 0.010% or less. Preferably, the S content is 0.008% or less.

O: 0% to 0.0040%

O is an element which is likely to form oxides and is an element which forms oxide-based inclusions having hard A1203 as a main composition and deteriorates drawability if O is present in the wire rod together with Al. Particularly, if the O content exceeds 0.0040%, oxide-based inclusions are coarsened even if the Al content restricted to the range described above, so that wire breaking frequently occurs during wire-drawing and drawability is significantly deteriorated. Therefore, the O content is restricted to 0.0040% or less. Preferably, the O content is 0.0030% or less, or 0.0025% or less.

Mo: 0% to 0.20%

Mo has an effect of enhancing the tensile strength of the drawn steel wire after wire-drawing. In order to achieve this effect, it is preferable that the Mo content is set to 0.02% or more. However, if the Mo content exceeds 0.20%, martensite structures are likely to be generated, so that there are cases where drawability is deteriorated. Therefore, the upper limit for the Mo content is set to 0.20%. More preferably, the upper limit for the Mo content is 0.10% or 0.07%. On the other hand, from the viewpoint of optimizing the balance between tensile strength and ductility of the wire rod or the drawn steel wire, it is more preferable that the lower limit for the Mo content is set to 0.04%.

B: 0% to 0.0030%

B has an effect of enhancing the balance between tensile strength and ductility of the drawn steel wire after wire-drawing. In order to achieve this effect, it is preferable that the B content is set to 0.0003% or more. However, if the B content exceeds 0.0030%, coarse BN is likely to be generated, so that there are cases where drawability is deteriorated. Therefore, the upper limit value for the B content is set to 0.0030%. More preferably, the upper limit for the B content is 0.0020% or 0.0015%. On the other hand, from the viewpoint of optimizing the balance between tensile strength and ductility of the wire rod or the drawn steel wire, it is more preferable that the lower limit for the B content is set to 0.0005%.

The remainder of the chemical composition of the wire rod includes Fe and impurities. Impurities are compositions incorporated from raw materials such as ore or scraps, or due to various causes during a manufacturing step when the steel is industrially manufactured. Impurities indicate substances allowed within a range in which the wire rod according to the present embodiment is not adversely affected.

<Area Fraction of Pearlite Structures and Residual Structures>

In order to stably prevent wire breaking and the like during primary wire-drawing, it is necessary that the area fraction of pearlite structures in the wire rod is 90% or more. The area fraction of pearlite may be set to 92% or more, 95% or more, 97% or more, or 100%. Moreover, the remainder (non-pearlitic region) of structures in the wire rod includes one or more of ferrite, cementite, bainite, and the like, and the area fraction of cementite is required to be less than 3%. It is preferable that the remainder of structures in the wire rod includes no martensite. However, it is allowable that martensite is contained up to approximately 0.2% by area fraction.

A drawn steel wire having high strength can be obtained from the wire rod having such metallographic structures with less frequent wire breaking at the time of wire-drawing. In the wire rod according to the present embodiment, ferrite in pearlite structures and ferrite structures in bainite structures are not counted as ferrite structures. In addition, in the wire rod according to the present embodiment, cementite structures are proeutectoid cementite which is transformed into the grain boundaries of prior austenites, cementite in pearlite structures and cementite structures in bainite structures are not counted as cementite structures. In order to set the metallographic structures in the wire rod within the range described above, the heating temperature before hot rolling, the finish temperature in hot rolling, the cooling rate after hot rolling, and the like are required to be preferably controlled.

<Maximum Grain Size of TiN>

In the case where Al was 0.002% or less and O was 0.0040% or less in the chemical composition of the wire rod, inclusions other than TiN, that is, sulfides or oxides were not observed in the origin of wire breaking. In the case where the maximum grain size of TiN measured by the method described below was 15 μm or more, wire breaking occurred during wire-drawing even though other requirements were satisfied. Therefore, in the wire rod according to the present embodiment, the maximum grain size of TiN is restricted to be less than 15 μm. The maximum grain size of TiN is preferably 12 μm or less and is more preferably 10 μm or less. There is no need to stipulate the lower limit value for the maximum grain size of TiN, and it may be set to 5 μm or 6 μm. In order to reduce the maximum grain size of TiN in the wire rod, it is necessary that at least the solidification rate is increased as much as possible when a slab which will become a material of the wire rod is manufactured.

<Method of Measuring Metallographic Structures>

Next, a method of measuring metallographic structures in the wire rod according to the present embodiment and the maximum grain size of TiN will be described.

The area fraction of pearlite structures is measured by the following method. First, a cross section of a wire rod (that is, a cutting surface having a right angle to a longitudinal direction of a wire rod) is subjected to mirror polishing. Thereafter, structures are manifested through picral etching to obtain observation samples. Next, ten points on an observed section of the sample are photographed at a magnification of 3,000-fold by using a field-emission scanning electron microscopy (FE-SEM). The observation points include five points in regions including a position at the ¼ depth of the radius of the wire rod from the surface of the wire rod, four points in regions including a position at the ½ depth of the radius of the wire rod from the surface of the wire rod, and one point in a region in a central portion of the wire rod. It is preferable that the measurement points are separated from one another as much as possible. The foregoing observation may be performed in a plurality of cross sections. The area per visual field is 5.0×10⁻⁴ mm² (20 μm in height and 25 μm in width). Subsequently, the area fraction of structures other than pearlite structures is obtained from the photograph by using an image analysis device which can analyze the shape, the area, and the like of grains. A value obtained by subtracting the area fraction (%) of those other than pearlite structures from 100% is adopted as the area fraction of pearlite structures. In addition, at this time, the area fractions of ferrite structures, cementite structures, bainite structures, and the like are also obtained by a similar method.

The maximum grain size of TiN is measured by the following method. First, a cross section perpendicular to a rolling direction (longitudinal direction of the wire rod) is cut out from the wire rod. The cross section is subjected to mirror polishing to be an observation sample, and TiN in this sample is observed by using an optical microscope. An observation is performed for each range of 2.5 mm×2.5 mm. The largest TiN within this observation range is photographed, and the area of the largest TiN is obtained through image analysis of the photograph. Then, ½ power of the area of the largest TiN is regarded as the grain size of the largest TiN. That is, in a case where the shape of the largest TiN within the observation range is regarded as a square, the length of one side of the square is regarded as the grain size of the largest TiN within the observation range. As described above, 20 cross sections are prepared for one sample, and this measurement is performed as many as 20 visual fields for one sample, thereby obtaining the maximum grain size of TiN in the observation range of 125 mm² in total.

The maximum grain size of TiN checked in the observation range of 125 mm² in total and the maximum grain size of TiN included in the entire wire rod have a strong correlationship. It is not possible to have the entire wire rod as the TiN-grain size measurement target. In addition, as for the wire rod in which the maximum grain size of TiN checked in the observation range of 125 mm² in total is within the range described above, the possibility of wire breaking occurring due to TiN at the time of wire-drawing is extremely low. Therefore, as for the wire rod of the present embodiment, the maximum grain size of TiN checked in the observation range of 125 mm² in total will be referred to as the maximum grain size of TiN in the wire rod.

In addition, in an optical microscope observation, TiN exhibits gold color. Therefore, TiN can be easily discriminated from other inclusions. In a case where there is an inclusion which is hardly identified based on color, an energy dispersive electron probe microscopy analyzer (EPMA) may be used to deteriminewhether the inclusion is TiN or not, which will be described below.

The observation range (2.5 mm×2.5 mm) described above is disposed at the center of a cross section of a wire rod. That is, the center of the observation range and the center of the cross section of the wire rod substantially coincide with each other. The reason is that the coarse TiN tends to be segregated near the central axis of the wire rod. In a case where the diameter of the wire rod is small and the observation range (2.5 mm×2.5 mm) described above cannot fit in a cross section of the wire rod, the observation range of TiN may be suitably reduced. In this case, the number of measurement cross sections may be increased to have the total area of the observation range of 125 mm² or more. In addition, it is desirable that the length of a diagonal line of the observation range of TiN is set to 90% or less than the diameter of the wire rod.

<Maximum Value of % Mn+2x % Cr in Central Portion 11 of Wire Rod>

The chemical composition of the wire rod according to the present embodiment (the average value of the chemical composition over the entirety of the wire rod) is as described above. However, there are cases where a chemical composition which can be obtained by performing local analysis on a cross section of a wire rod using an EPMA or the like slightly differs from the average value of the chemical composition of the wire rod due to the influences of segregation of alloy compositions and precipitation of inclusions. Since segregation of alloying elements is suppressed within a predetermined range in the wire rod according to the present embodiment, the wire rod has the features described below.

In the wire rod 1 according to the present embodiment, the region from the central axis to 1/10 of the diameter of the wire rod is defined as the central portion 11. The maximum value of % Mn+2x % Cr (hereinafter, which may be disclosed as Mn+2Cr) measured on a cutting surface having a right angle to a longitudinal direction of the wire rod in a region where the S content and the O content are less than 1% in the central portion 11 is 2.0 times or less than the average value of % Mn+2x % Cr over the entirety of the wire rod. Here, when the segregations of Mn and Cr are evaluated, it is necessary to exclude the influence of sulfides and oxides. In the regions of the sulfides and the oxides, the value of the S content or the O content significantly exceeds 1%. Therefore, in the wire rod according to the present embodiment having the chemical composition described above, the region where the S content and the O content are less than 1% can be regarded as the region where the sulfides and the oxides are not present. Hereinafter, there are cases where % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where the S content and the O content are less than 1% in the central portion 11 is simply disclosed as “Mn+2Cr in the central portion 11”.

If the maximum value of Mn+2Cr in the central portion 11 exceeds 2.0 times of Mn+2Cr in the entire wire rod 1, deformability of the central portion 11 is remarkably deteriorated. As a result, wire breaking is likely to occur during wire-drawing. Therefore, the maximum value of Mn+2Cr in the central portion 11 is set to 2.0 times or less than Mn+2Cr in the entirety. The maximum value of Mn+2Cr in the central portion 11 is preferably set to 1.7 times or less and is more preferably set to 1.5 times or less than Mn+2Cr in the entirety. Here, as described above, in this specification, % Mn and % Cr respectively indicate the Mn content and the Cr content by mass %. Although, it is not necessary that the lower limit value for the ratio of the maximum value of Mn+2Cr in the central portion 11 to Mn+2Cr in the entirety is stipulated, it may be set to 1.2, 1.4, or 1.5. Hereinafter, there are cases where the ratio of the maximum value of Mn+2Cr in the central portion 11 to the average value of Mn+2Cr in the entire wire rod 1 (the maximum value of Mn+2Cr in the central portion/the average value of Mn+2Cr in the entire wire rod) is disclosed as a “segregation amount of Mn+2Cr in the central portion”.

<Ratio of Maximum Value to Minimum Value (Maximum Value/Minimum Value) of % Mn+2x % Cr in Outer Circumferential Portion 12 of Wire Rod>

In the wire rod according to the present embodiment, a region from the outer edge of the central portion to the depth of 0.1 mm from the surface is defined as the outer circumferential portion 12. The ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value to the minimum value (that is, the maximum value/the minimum value) in the region where the S content and the O content are less than 1% in the outer circumferential portion 12 is 2.0 or less. In the outer circumferential portion 12 as well, similar to the central portion 11, when the segregations of Mn and Cr are evaluated, it is necessary to exclude the influence of sulfides and oxides. Therefore, regions where the S content or the O content exceeds 1% are excluded, and the value of % Mn+2x % Cr in the region where both the S content and the O content are less than 1% is measured. Hereinafter, there are cases where % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the O content are less than 1% in the outer circumferential portion 12 is simply disclosed as “Mn+2Cr in the outer circumferential portion 12”.

If the ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 exceeds 2.0, Mn and Cr are segregated in a band shape and resulting in a hard structure in the wire rod. Therefore, cracking proceeds along the region due to wire-drawing, so that wire breaking is likely to occur. Accordingly, the ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 is set to 2.0 or less. The ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 is preferably set to 1.6 or less and is more preferably set to 1.4 or less. In order to resolve segregation of Mn and Cr in the central portion 11 and the outer circumferential portion 12 of the wire rod, it is required that at least electromagnetic stirring is performed when a slab for a material of the wire rod is manufactured, the solidification rate is increased as much as possible, and the slab and a steel billet are retained at a sufficiently high temperature for a long time. Although, it is not necessary that the lower limit value for the ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 is stipulated, it may be set to 1.3, 1.4, or 1.5. Hereinafter, there are cases where the ratio of the maximum value to the minimum value of Mn+2Cr (the maximum value of Mn+2Cr / the minimum value of Mn+2Cr) in the outer circumferential portion 12 is disclosed as a “segregation amount of Mn+2Cr in the outer circumferential portion”.

<Method of Measuring Concentrations of Mn and Cr in Central Portion 11 and Outer Circumferential Portion 12>

The concentration of Mn and the concentration of Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the O content are less than 1% in the central portion 11 of the wire rod are measured by the following method. First, cross sections (cutting surfaces having the right angle to the longitudinal direction) are cut out at five points from the wire rod at intervals of 200 mm in length. Next, as shown in FIG. 1, in a linear analysis region 13 extending from the center on each of the cutting surfaces to 1/10 of a diameter D, each of the elements Mn, Cr, S, and O is subjected to linear analysis using an energy dispersive electron probe micro analyzer (EPMA), and the concentration distribution of each of the elements in the linear analysis region 13 on each of the cutting surfaces is measured. It is preferable that linear analysis using an EPMA is performed at an accelerated voltage of 15 kV, a beam diameter of 1 μm, a scanning rate of 200 μm/min, and intervals of the measurement points of 2 μm.

Next, measurement results of the region where 1% or more of S is present and/or the region where 1% or more of O is present are excluded from the measurement results of the obtained linear analysis of the concentrations of Mn and Cr. In this operation, the influence of the oxides and the sulfides (inclusions) can be excluded from the evaluation results of segregation. In the linear analysis region 13 on each of the cutting surfaces excluding the region where 1% or more of S is present and/or the region where 1% or more of O is present, the maximum value of % Mn+2x % Cr is obtained, and the maximum values of % Mn+2x % Cr in the linear analysis regions 13 on the cutting surfaces at five points are adopted as the maximum value of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the O content are less than 1% in the central portion 11.

The average value of % Mn+2x % Cr over the entirety of the wire rod may be calculated based on the chemical composition of the wire rod (average value of the chemical composition over the entirety of the wire rod). That is, in a case where the material of the wire rod is already known, the average value of % Mn+2x % Cr over the entirety of the wire rod may be calculated based on the amount of Mn and the amount of Cr included in the material of the wire rod. In a case where the material of the wire rod is uncertain, the average value of the Mn content and the average value of the Cr content over the entirety of the wire rod may be obtained by an ordinary chemical composition analysis method, and the average value of % Mn+2x % Cr over the entirety of the wire rod may be calculated based on the average values thereof.

The value of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the O content are less than 1% in the outer circumferential portion 12 of the wire rod is measured by the following method. First, similar to measuring the concentrations of Mn and Cr in the central portion 11, cross sections (cutting surfaces having the right angle to the longitudinal direction) are cut out at five points from the wire rod at intervals of 200 mm in length. Next, as shown in FIG. 2, in a linear analysis region 14 extending from a point out of the center of the cross section of the wire rod by 1/10 of the diameter D (that is, the outer edge of the central portion 11) to a point out of the outer edge of the cross section of the wire rod to the depth of 0.1 mm, along a straight line passing through the center of the cross section of the wire rod, each of the elements Mn, Cr, S, and O is subjected to linear analysis using an EPMA, similar to measuring the concentrations of Mn, Cr, S, and O in the central portion 11, and the concentration distribution of each of the elements in the linear analysis region 14 on each of the cutting surfaces is measured.

Next, the measurement results of the region where 1% or more of S is present and/or the region where 1% or more of O is present are excluded from the obtained results of the linear analysis of the concentrations of Mn and Cr. In this operation, the influence of oxides and sulfides (inclusions) can be excluded from the evaluation results of segregation. In the linear analysis region 14 on each of the cutting surfaces excluding the region where 1% or more of S is present and/or the region where 1% or more of O is present, the maximum value of % Mn+2x % Cr is obtained, and the maximum value of % Mn+2x % Cr in the linear analysis region 14 on the cutting surfaces at five points are adopted as the maximum value of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the O content are less than 1% in the outer circumferential portion 12. In addition, in the linear analysis region 14 on each of the cutting surfaces excluding the region where 1% or more of S is present and/or the region where 1% or more of O is present, the minimum value of the concentration of % Mn+2x % Cr is obtained, and the minimum value of the concentration of % Mn+2x % Cr in the linear analysis region 14 on the cutting surfaces at five points are adopted as the minimum value of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the O content are less than 1% in the outer circumferential portion 12. The ratio of the maximum value to the minimum value (the maximum value/the minimum value) is calculated using the results of the maximum value and the minimum value of % Mn+2x % Cr obtained in this manner, and the calculated result is adopted as the ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value to the minimum value (the maximum value / the minimum value) in the region where the S content and the O content are less than 1% in the outer circumferential portion 12.

The diameter of the wire rod according to the present embodiment is not particularly limited. The diameter of wire rods currently distributed in the market is within a range of 3.6 mm to 8.0 mm in many cases. Therefore, the diameter of the wire rod according to the present embodiment may be set to 3.6 mm to 8.0 mm.

<Manufacturing Method>

Next, a method of manufacturing the wire rod according to the present embodiment will be described.

In a case where the wire rod according to the present embodiment is manufactured, the conditions of steps and processes may be set in accordance with the composition, the target performance, the wire diameter, and the like of a steel such that the area fraction of pearlite, structures other than pearlite, and the maximum grain size of TiN reliably satisfy each of the conditions described above.

In a case where a small amount of a steel is cast for an experiment, if weight of the steel is 150 kg or less, a raw material is melted first and evacuation is performed for 20 minutes or more thereafter. Casting is performed by using a mold of which the average cross-sectional area inside thereof is 120 cm² or less, and an ingot is obtained. Examples of materials of a mold used when obtaining an ingot include cast iron. Examples of not-preferable materials include silica. In addition, 15% by volume fraction of both ends of an ingot in the longitudinal direction after casting are not used, and the corresponding parts of the ingot are cut before hot forging is performed.

Next, the ingot from which both ends are removed is heated at a temperature of 1,260° C. to 1,300° C. for 8 hours to 12 hours and is cooled to 500° C. or less in a furnace. Subsequently, the ingot is heated to a temperature of 1,200° C. to 1,250° C. and is subjected to hot forging thereafter, and a steel billet is obtained.

In a case where the wire rod according to the present embodiment is manufactured by a manufacturing method including continuous casting, after a molten steel is smelted by using a converter, electromagnetic stirring of the molten steel is sufficiently performed, the average cooling rate is set to 5 ° C./min or more from the start of solidification to the end of solidification, and reduction is performed in the middle of solidification to obtain a slab.

Next, the cast slab is heated at a temperature of 1,260° C. to 1,300° C. for 8 hours to 12 hours and is cooled to 500° C. or less in the furnace. Subsequently, the slab is heated at a temperature of 1,200° C. to 1,250° C. for 4 hours to 6 hours and is subjected to blooming thereafter, and a steel billet is obtained.

The steel billet manufactured by either method described above is heated to be 1,050° C. to 1,150° C. and is retained within this temperature range for 40 minutes to 60 minutes. Thereafter, hot rolling is performed at the finish rolling temperature of 900° C. to 1,000° C. Although, the diameter of the wire rod after hot rolling is not particularly limited, the diameter is set to 3.6 mm to 8.0 mm in many cases as described above. Immediately after the end of finish rolling, the finish-rolled wire rod is cooled through cooling in which water cooling and wind cooling in the atmosphere are combined (primary cooling) until the wire rod reaches a temperature range of 680° C. to 730° C. at the average cooling rate of 30 ° C./sec or more. Thereafter, the wire rod is cooled through wind cooling in the atmosphere (secondary cooling) until the wire rod reaches a temperature range of 610° C. to 650° C. at the average cooling rate of 10 ° C./sec to 20 ° C./sec. Thereafter, air cooling (third cooling) is performed until the wire rod reaches a temperature of 500° C. or less. The wire rod according to the present embodiment is manufactured by the method described above. The wire rod obtained in this manner is a so-called hot-rolled wire rod. However, the wire rod further subjected to cold working such as cold rolling and wire-drawing, in addition thereto is also regarded as the wire rod according to the present embodiment as long as the wire rod satisfies the requirements described above.

In this specification, the heating temperature of a steel billet indicates the surface temperature of a steel billet. The finish rolling temperature indicates the surface temperature of the wire rod immediately after finish rolling. The cooling rate after finish rolling indicates the cooling rate on the surface of the wire rod. In addition, the average cooling rate in primary cooling where water cooling and wind cooling in the atmosphere are combined is the value obtained by dividing the difference between the surface temperature of the wire rod at the start point of time of spraying water or the atmosphere to the wire rod and the surface temperature of the wire rod at the end point of time of spraying water or the atmosphere to the wire rod by the spraying time. The average cooling rate in wind cooling in the atmosphere (secondary cooling) is the value obtained by dividing the difference between the surface temperature of the wire rod at the start point of time of spraying the atmosphere to the wire rod and the surface temperature of the wire rod at the end point of time of spraying the atmosphere to the wire rod by the spraying time. As long as the conditions of the cooling rate described above are satisfied, a coolant to be sprayed to the wire rod in primary cooling and secondary cooling is not limited to water or the atmosphere.

As described above, the wire rod of the present embodiment has a predetermined composition and includes metallographic structures in which the area fraction of pearlite structures is 90% or more. The remainder includes one or more of ferrite, cementite, and bainite. The area fraction of the cementite is less than 3%. The maximum grain size of TiN is less than 15 μm. The maximum value of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in the region where both the S content and the 0 content are less than 1% in the central portion 11, which is the region from the central axis of the wire rod to 1/10 of the diameter of the wire rod, is 2.0 times or less than the average value of % Mn+2x % Cr over the entirety of the wire rod. The ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value to the minimum value (the maximum value/the minimum value) in the region where both the S content and the O content are less than 1% in the outer circumferential portion 12, which is the region from the outer edge of the central portion to a depth of 0.1 mm from the surface (outer circumferential surface), is 2.0 or less. Therefore, the wire rod of the present embodiment has excellent drawability, so that a drawn steel wire having high strength can be manufactured with less frequent wire breaking when wire-drawing is performed to obtain a drawn steel wire.

Next, Examples of the present invention will be described. However, conditions of the Examples are merely examples of conditions employed to check for the feasibility and the effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can employ various conditions as long as the present invention without departing from the gist of the present invention and the object of the present invention is achieved.

EXAMPLES

Steels A1 to Z1, A2, and G2 having the composition (chemical composition) shown in Table 1 were cast into ingots having the average cross-sectional area shown in Table 1 and having a weight of 50 kg or 150 kg. When the ingots were obtained, molds formed of the mold materials shown in Table 1 were used. In addition, by the volume fraction shown in Table 1, both ends of the ingots in the longitudinal direction were cut and removed.

Next, the ingots from which both ends were removed were subjected to heat treatment under the heat treatment condition (ingot heat treatment condition) shown in Table 1 and were cooled to 400° C. in the furnace. Subsequently, the ingots were heated to 1,230° C. to obtain steel billets having a diameter of 80 mm through hot forging, and the steel billets were subjected to air cooling to the room temperature.

In addition, steels B2 to F2 having the composition (chemical composition) shown in Table 2 were smelted by the converter and were subjected to continuous casting thereafter. When casting was performed, electromagnetic stirring of molten steels was sufficiently performed, and reduction was performed in the middle of solidification at the average cooling rate of 6 ° C./min from the start of solidification to the end of solidification. Next, cast slabs were subjected to heat treatment under the heat treatment conditions (slab heat treatment conditions) shown in Table 2 and were subjected to air cooling to 500° C. or less in the furnace. Subsequently, the slabs were heated under the conditions shown in Table 2. Thereafter, steel billets of 122 mm square were obtained through blooming.

TABLE 1 Chemical composition, mass (%) Remainder: Fe and impurities Mn + Steel Classification C Si Mn P S Cr Mo Al Ti N B 0 2Cr A1 Comparison 0.84 0.21 0.41 0.008 0.005 0.18 — 0.001 0.001 0.0038 — 0.0024 0.77 B1 Invention 0.92 0.31 0.61 0.009 0.006 0.18 — 0.001 0.002 0.0035 — 0.0022 0.97 C1 Comparison 0.93 0.23 0.25 0.010 0.005 0.11 — 0.001 0.001 0.0032 — 0.0018 0.47 D1 Invention 0.93 0.22 0.26 0.009 0.007 0.14 — 0.002 0.002 0.0042 — 0.0019 0.54 E1 Comparison 0.93 0.20 0.45 0.011 0.006 0.06 — 0.001 0.001 0.0039 — 0.0019 0.57 F1 Comparison 1.02 0.22 0.45 0.012 0.008 0.31 — 0.002 0.001 0.0040 — 0.0016 1.07 G1 Invention 1.07 0.21 0.31 0.007 0.006 0.21 — 0.001 0.002 0.0038 — 0.0018 0.73 H1 Comparison 1.00 0.22 0.28 0.009 0.005 0.48 — 0.001 0.001 0.0031 — 0.0016 1.24 I1 Invention 1.00 0.22 0.21 0.008 0.004 0.39 — 0.001 0.001 0.0035 — 0.0014 0.99 J1 Comparison 1.22 0.20 0.31 0.006 0.005 0.20 — 0.001 0.001 0.0035 — 0.0017 0.71 K1 Comparison 1.02 0.21 0.32 0.007 0.005 0.21 — 0.003 0.001 0.0032 — 0.0014 0.74 L1 Comparison 1.01 0.23 0.31 0.006 0.006 0.20 — 0.001 0.003 0.0039 — 0.0016 0.71 M1 Comparison 1.02 0.20 0.30 0.007 0.007 0.19 — 0.001 0.002 0.0062 — 0.0017 0.68 N1 Comparison 1.02 0.22 0.29 0.008 0.005 0.20 — 0.002 0.001 0.0041 — 0.0045 0.69 O1 Comparison 1.03 0.21 0.32 0.007 0.006 0.21 — 0.001 0.002 0.0040 — 0.0015 0.74 P1 Comparison 1.02 0.19 0.31 0.009 0.005 0.20 — 0.002 0.002 0.0042 — 0.0016 0.71 Q1 Comparison 1.01 0.23 0.32 0.007 0.004 0.21 — 0.001 0.002 0.0039 — 0.0017 0.74 R1 Comparison 1.02 0.22 0.29 0.008 0.006 0.20 — 0.002 0.002 0.0040 — 0.0019 0.69 S1 Invention 1.01 0.21 0.28 0.008 0.005 0.21 0.05 0.001 0.001 0.0037 — 0.0018 0.70 T1 Invention 1.02 0.21 0.32 0.009 0.007 0.23 — 0.001 0.001 0.0036 0.0013 0.0018 0.78 U1 Invention 1.01 0.20 0.31 0.007 0.006 0.22 0.07 0.001 0.001 0.0034 0.0014 0.0016 0.75 V1 Comparison 1.02 0.22 0.64 0.012 0.008 0.21 — 0.002 0.001 0.0040 — 0.0016 1.06 W1 Invention 1.03 0.50 0.32 0.007 0.005 0.20 — 0.001 0.001 0.0035 — 0.0014 0.72 X1 Comparison 1.02 0.22 0.42 0.008 0.007 0.22 — 0.002 0.001 0.0042 — 0.0015 0.86 Y1 Comparison 1.03 0.21 0.41 0.009 0.006 0.21 — 0.001 0.002 0.0043 — 0.0019 0.83 Z1 Comparison 1.02 0.21 0.43 0.008 0.006 0.23 — 0.001 0.002 0.0037 — 0.0017 0.89 A2 Comparison 1.07 0.21 0.31 0.009 0.007 0.21 — 0.002 0.001 0.0041 — 0.0016 0.73 G2 Invention 1.01 0.80 0.31 0.006 0.005 0.15 — 0.001 0.001 0.0031 — 0.0015 0.61 Conditions of casting Volume Evacuation Ingot Average cross- fraction with time weight Material sectional area cutting both ends Heat treatment Steel (min) (kg) of mold (cm²) (%) conditions of ingot A1 25 50 Cast iron 115 18 1,290° C. × 8 hrs. B1 25 50 Cast iron 115 18 1,280° C. × 8 hrs. C1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. D1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. E1 25 50 Cast iron 115 18 1,280° C. × 8 hrs. F1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. G1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. H1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. I1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. J1 25 50 Cast iron 115 18 1,260° C. × 10 hrs. K1 25 50 Cast iron 115 18 1,260° C. × 8 hrs. L1 25 50 Cast iron 115 18 1,260° C. × 8 hrs. M1 25 50 Cast iron 115 18 1,260° C. × 8 hrs. N1 10 50 Cast iron 115 18 1,260° C. × 8 hrs. O1 25 150 Cast iron 200 18 1,260° C. × 8 hrs. P1 25 50 Silica 115 18 1,260° C. × 8 hrs. Q1 25 50 Cast iron 140 18 1,260° C. × 8 hrs. R1 25 50 Cast iron 115 5 1,260° C. × 8 hrs. S1 25 50 Cast iron 115 18 1,260° C. × 12 hrs. T1 25 50 Cast iron 115 18 1,260° C. × 12 hrs. U1 25 50 Cast iron 115 18 1,260° C. × 12 hrs. V1 25 50 Cast iron 115 18 1,260° C. × 8 hrs. W1 25 50 Cast iron 115 18 1,260° C. × 12 hrs. X1 25 50 Cast iron 115 18 None Y1 25 50 Cast iron 115 18 1,200° C. × 10 hrs. Z1 25 50 Cast iron 115 18 1,260° C. × 4 hrs. A2 25 50 Cast iron 115 18 1,200° C. × 10 hrs. G2 25 50 Cast iron 115 18 1,260° C. × 12 hrs. “Invention” in the classification field indicates that the chemical composition is within the range of the present invention and the manufacturing conditions are appropriate. “Comparison” expresses that the chemical composition is beyond the range of the present invention and the method of manufacturing the steel billet is inappropriate. The underline indicates that the value is beyond the range of the present invention.

TABLE 2 Chemical composition, mass (%) Remainder: Fe and impurities Steel Classification C Si Mn P S Cr Mo Al Ti N B2 Invention 1.08 0.23 0.29 0.005 0.005 0.19 — 0.001 0.001 0.0037 C2 Invention 1.02 0.22 0.31 0.008 0.006 0.21 — 0.001 0.002 0.0029 D2 Invention 0.97 0.41 0.41 0.006 0.007 0.26 — 0.001 0.001 0.0036 E2 Comparison 1.03 0.21 0.38 0.008 0.006 0.22 — 0.001 0.002 0.0029 F2 Comparison 1.02 0.20 0.40 0.009 0.007 0.20 — 0.001 0.001 0.0035 Heat Heating Chemical composition, mass (%) treatment conditions Remainder: Fe and impurities conditions of slab Steel B O Mn + 2Cr of slab during blooming B2 — 0.0021 0.67 1,260° C. × 8 hrs. 1,250° C. × 5 hrs. C2 0.0012 0.0020 0.73 1,270° C. × 10 hrs. 1,250° C. × 6 hrs. D2 — 0.0019 0.93 1,270° C. × 8 hrs. 1,250° C. × 4 hrs. E2 0.0012 0.0020 0.82 1,200° C. × 4 hrs. 1,250° C. × 4 hrs. F2 — 0.0023 0.80 1,260° C. × 8 hrs. 1,200° C. × 2 hrs. “Invention” in the classification field indicates that the chemical composition is within the range of the present invention and the manufacturing conditions are appropriate. “Comparison” expresses that the chemical composition is beyond the range of the present invention and the method of manufacturing the steel billet is inappropriate. The underline indicates that the value is beyond the range of the present invention.

The steel billets manufactured by the method described above were heated to the steel billet heating temperatures shown in Table 3. The temperature was retained at the same heating temperature during the steel billet heating retention time shown in Table 3, and a hot rolling step was performed at the finish rolling temperature shown in Table 3 such that the finish rolling diameter (diameter) becomes 5.5 mm. After finish rolling, the wire rods were cooled to 700° C. at the average cooling rates shown in Table 3 through cooling (primary cooling) in which water cooling and wind cooling in the atmosphere were combined. Thereafter, the wire rods were cooled from 700° C. to 610° C. at the average cooling rates shown in Table 3 through wind cooling in the atmosphere (secondary cooling). The average cooling rates (primary cooling rate) to 700° C. and the average cooling rate (secondary cooling rate) from 700° C. to 610° C. regarding each of the samples were adopted as shown in the table. Thereafter, the wire rods at a temperature less than 610° C. were subjected to air cooling (third cooling), and the wire rods were obtained.

TABLE 3 Average Steel billet Average cooling Cooling Steel billet heating Finish Finish cooling rate to method to heating retention rolling rolling rate to 700° C. to temperature Test temperature time temperature diameter 700° C. 610° C. less than No. Steel (° C.) (min) (° C.) (mm) (° C./sec) (° C./sec) 610° C. 1 A1 1100 50 930 5.5 55 13 air cooling 2 B1 1100 50 930 5.5 55 13 air cooling 3 C1 1100 50 930 5.5 60 15 air cooling 4 D1 1100 50 930 5.5 60 15 air cooling 5 E1 1100 50 930 5.5 60 15 air cooling 6 F1 1100 50 930 5.5 60 15 air cooling 7 G1 1100 50 930 5.5 65 17 air cooling 8 H1 1100 50 930 5.5 65 17 air cooling 9 I1 1100 50 930 5.5 65 17 air cooling 10 J1 1100 50 930 5.5 65 17 air cooling 11 K1 1100 50 930 5.5 60 15 air cooling 12 L1 1100 50 930 5.5 60 15 air cooling 13 M1 1100 50 930 5.5 60 15 air cooling 14 N1 1100 50 930 5.5 60 15 air cooling 15 O1 1100 50 930 5.5 60 15 air cooling 16 P1 1100 50 930 5.5 60 15 air cooling 17 Q1 1100 50 930 5.5 60 15 air cooling 18 R1 1100 50 930 5.5 60 15 air cooling 19 S1 1100 50 930 5.5 60 15 air cooling 20 T1 1100 50 930 5.5 60 15 air cooling 21 U1 1100 50 930 5.5 60 15 air cooling 22 V1 1100 50 930 5.5 60 15 air cooling 23 W1 1100 50 930 5.5 60 15 air cooling 24 X1 1100 50 930 5.5 60 15 air cooling 25 Y1 1100 50 930 5.5 60 15 air cooling 26 Z1 1100 50 930 5.5 60 15 air cooling 27 A2 1100 50 930 5.5 60 15 air cooling 28 B2 1100 50 850 5.5 60 15 air cooling 29 B2 1100 50 930 5.5 20 15 air cooling 30 B2 1100 50 930 5.5 60 8 air cooling 31 B2 1100 50 930 5.5 60 12 air cooling 32 B2 1100 50 930 5.5 60 18 air cooling 33 C2 1100 50 930 5.5 60 30 air cooling 34 C2 1230 40 1,080 5.5 60 15 air cooling 35 C2 1050 60 910 5.5 60 15 air cooling 36 C2 1150 40 970 5.5 60 15 air cooling 37 C2 1100 50 930 5.5 50 15 air cooling 38 D2 1100 50 930 5.5 50 12 air cooling 39 D2 1100 50 930 5.5 65 3 air cooling 40 D2 1100 50 930 5.5 65 25 air cooling 41 D2 1100 50 930 5.5 65 17 air cooling 42 D2 1100 50 930 5.5 60 15 air cooling 43 E2 1100 50 930 5.5 60 15 air cooling 44 F2 1100 50 930 5.5 65 15 air cooling 45 B2 1100 15 930 5.5 65 15 air cooling 46 G2 1100 50 930 5.5 60 15 air cooling

The measuring method described above was performed with respect to the obtained wire rods to obtain the area fractions of pearlite structures, ferrite structures, cementite structures, and bainite structures; the segregation amount of Mn+2Cr in the central portion (that is, the ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value in the region where both the S content and the 0 content were less than 1% in the central portion to the average value of % Mn+2x % Cr over the entirety of the wire rod); the segregation amount of Mn+2Cr in the outer circumferential portion (that is, the ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value to the minimum value in the region where the S content and the O content were less than 1% in the outer circumferential portion); and the maximum grain size of TiN. Table 4 and Table 5 show the results thereof.

TABLE 4 Segregation Area Area Area Area Maximum amount of fraction of fraction of fraction of fraction of grain size Mn + 2Cr in Test pearlite ferrite cementite bainite of TiN central No. Classification Steel (%) (%) (%) (%) (μm) portion 1 Comparison *A1 97 2 0 1 8 1.6 2 Invention B1 95 0 0 5 11 1.7 3 Comparison *C1 98 0 0 2 9 1.4 4 Invention D1 97 0 0 3 12 1.6 5 Comparison *E1 98 1 0 1 7 1.5 6 Comparison *F1 92 0 0 7 8 1.6 7 Invention G1 97 0 1 2 10 1.4 8 Comparison *H1 *87 0 0 11 6 1.7 9 Invention I1 92 0 0 8 6 1.6 10 Comparison *J1 95 0 *4 2 7 1.5 11 Comparison *K1 98 0 0 2 7 1.4 12 Comparison *L1 98 0 0 2 *16 1.6 13 Comparison *M1 98 0 0 2 *18 1.5 14 Comparison *N1 98 0 0 2 9 1.5 15 Comparison *O1 97 0 0 3 *18 1.6 16 Comparison *P1 98 0 0 2 *17 *2.1 17 Comparison *Q1 96 0 0 4 *15 1.5 18 Comparison *R1 98 0 0 2 *20 1.5 19 Invention S1 94 0 0 6 7 1.6 20 Invention T1 96 0 0 4 6 1.5 Segregation amount The number of times The number of times of Mn + 2Cr in outer of wire breaking of wire breaking Tensile Test circumferential ϕ5.5 → ϕ1.10 ϕ1.5 → ϕ0.19 strength No. portion (times/10 kg) (times/18 kg) (MPa) Remarks 1 1.6 0 0 #3940  2 1.5 0 0 4136 3 1.5 0 0 #4057  4 1.5 0 0 4183 5 1.5 0 0 #4046  6 1.4 #2 #1  4527 * martensite structure included 7 1.4 0 0 4649 8 1.6 #3 # twice (drawing — * martensite suspended) structure included 9 1.6 0 0 4586 10 1.4 #3 # twice (drawing — suspended) 11 1.5 0 #1  4492 12 1.4 0 #1  4513 13 1.6 0 # twice (drawing — suspended) 14 1.6 #1 # twice (drawing — suspended) 15 *2.1 #1 # twice (drawing — suspended) 16 1.8 #1 #1  4525 17 1.7 0 #1  4498 18 1.4 0 # twice (drawing — suspended) 19 1.5 0 0 4620 20 1.4 0 0 4593 “Invention” in the classification field expresses the example of the present invention, and “Comparison” expresses the comparative example. The symbol * indicates that the value is beyond the conditions stipulated by the present invention. The symbol # indicates that the value is inferior to the target performance of the present invention.

TABLE 5 Segregation Area Area Area Area Maximum amount of fraction of fraction of fraction of fraction of grain size Mn + 2Cr Test pearlite ferrite cementite bainite of TiN in central No. Classification Steel (%) (%) (%) (%) (μm) portion 21 Invention U1 91 0 0 9 6 1.4 22 Comparison *V1 93 0 0 6 7 1.6 23 Invention W1 95 0 0 5 7 1.5 24 Comparison *X1 95 0 1 4 9 *2.7 25 Comparison *Y1 97 0 0 3 8 *2.4 26 Comparison *Z1 98 0 0 2 9 *2.1 27 Comparison *A2 97 0 0 3 9 *2.4 28 Comparison B2 95 0 *4 1 7 1.4 29 Comparison B2 95 0 *3 2 7 1.5 30 Comparison B2 97 0 *3 0 7 1.7 31 Invention B2 97 0 2 1 7 1.6 32 Invention B2 97 0 1 2 7 1.6 33 Comparison C2 *86 0 0 14 9 1.7 34 Comparison C2 *88 0 0 12 9 1.6 35 Invention C2 99 0 0 1 9 1.6 36 Invention C2 95 0 0 5 9 1.6 37 Invention C2 98 0 0 2 9 1.5 38 Invention D2 98 0 0 2 7 1.5 39 Comparison D2 97 0 *3 0 7 1.5 40 Comparison D2 *87 0 0 13 7 1.5 41 Invention D2 95 0 0 5 7 1.5 42 Invention D2 97 0 0 3 7 1.5 43 Comparison *E2 97 0 0 3 10 *2.2 44 Comparison *F2 98 0 0 2 9 *2.1 45 Comparison B2 96 0 1 3 7 1.8 46 Invention G2 94 0 0 6 8 1.5 Segregation amount The number of times The number of times of Mn + 2Cr in outer of wire breaking of wire breaking Tensile Test circumferential ϕ5.5 → ϕ1.10 ϕ1.5 → ϕ0.19 strength No. portion (times/10 kg) (times/18 kg) (MPa) Remarks 21 1.6 0 0 4641 22 1.5 #2 #1  4528 *martensite structure included 23 1.4 0 0 4624 24 *3.0 #3 # twice (drawing — suspended) 25 *2.1 #1 # twice (drawing — suspended) 26 1.8 #1 # twice (drawing — suspended) 27 *2.1 #2 # twice (drawing — suspended) 28 1.3 #3 #1  4634 29 1.6 #2 0 4628 30 1.3 #2 0 4625 31 1.4 0 0 4619 32 1.5 0 0 4633 33 1.3 #1 0 4523 34 1.4 #1 0 4519 35 1.5 0 0 4520 36 1.4 0 0 4509 37 1.5 0 0 4521 38 1.6 0 0 4397 39 1.6 #3 0 4401 40 1.5 #1 0 4386 41 1.6 0 0 4392 42 1.5 0 0 4389 43 *2.1 #1 # twice (drawing — suspended) 44 1.9 0 #1  4512 45 *2.2 0 #1  4615 46 1.4 0 0 4633 “Invention” in the classification field expresses the example of the present invention, and “Comparison” expresses the comparative example. The symbol *indicates that the value is beyond the conditions stipulated by the present invention. The symbol #indicates that the value is inferior to the target performance of the present invention.

The wire rod was subjected to descaling and lubricating treatment by an ordinary method. Thereafter, dry wire-drawing was performed at a pass schedule in which the reduction of area in each die became 18% on average. The wire rod of 18 kg was subjected to wire-drawing from the diameter of 5.5 mm to the diameter of 1.50 mm, and the wire rod of 10 kg was subjected to wire-drawing from the diameter of 5.5 mm to the diameter of 1.10 mm Table 4 and Table 5 show the number of times of wire breaking in the wire rods in the case where wire-drawing was performed from the diameter of 5.5 mm to the diameter of 1.10 mm. In the wire rods subjected to wire-drawing from the diameter of 5.5 mm to the diameter of 1.50 mm, primary drawability was not evaluated, and secondary drawability (which will be described below) was evaluated. In the case where wire breaking did not occur even once when dry wire-drawing was performed from the diameter of 5.5 mm to the diameter of 1.10 mm, primary drawability was evaluated to be favorable. The true strain when drawn from the diameter of 5.5 mm to the diameter of 1.10 mm was 3.22.

Here, the true strain (ε) is expressed by the following Expression (i) using the diameter (d₀) before wire-drawing and the diameter (d) of the drawn steel wire after wire-drawing.

ε=2 ln(d ₀ /d)   (i)

Subsequently, the drawn wire rods (drawn steel wires) having the diameter of 1.50 mm described above were subjected to patenting treatment using a heating furnace and a lead bath furnace. The heating furnace was set such that the temperature of the drawn steel wires was retained at 975° C. to 990° C. for 5 seconds to 15 seconds. In addition, the temperature of the lead bath was set to 585° C. to 595° C., and the immersion time in the lead bath was set to 7 seconds to 10 seconds. The drawn steel wires after patenting were successively subjected to brass coating by an ordinary method.

The drawn steel wires after brass coating were subjected to wet wire-drawing (final wire-drawing) to the diameter of 0.19 mm at the pass schedule in which the reduction of area in each die became 15% on average. The true strain when drawn from the diameter of 1.50 mm to the diameter of 0.19 mm was 4.13. In this wet wire-drawing (final wire-drawing), drawability was evaluated, and Table 4 and Table 5 show the results thereof. When the wire rods of 18 kg were subjected to wet wire-drawing from the diameter of 1.50 mm to the diameter of 0.19 mm, in the case where the number of times of wire breaking was zero, secondary drawability was evaluated to be favorable. On the other hand, in the case where the number of times of wire breaking was one or more, drawability was evaluated to be poor. At the point of time the number of times of wire breaking became two, evaluation of wire-drawing to the diameter of 0.19 mm and thereafter was suspended.

The strength of the drawn steel wires after wet wire-drawing was investigated as follows. A tension test was performed on three drawn steel wires, which could be drawn to the diameter of 0.19 mm, and the tensile strength was measured. Table 4 and Table 5 show the average values of the tensile strength of the three drawn steel wires.

The target performance of the wire rod of the present invention is as follows. The number of times of wire breaking when the wire rod of 10 kg is subjected to dry wire-drawing to achieve the true strain of 3.22 is zero. The number of times of wire breaking is zero when the drawn steel wire of 18 kg after patenting and brass coating is subjected to wet wire-drawing to achieve the true strain of 4.13. In addition, the tensile strength of the drawn steel wire having the diameter of 0.19 mm is 4,100 MPa or more.

From Table 4 and Table 5, it is clear that the test numbers which satisfy all of the conditions stipulated in the present invention satisfy all the performance described above. In addition, some test numbers show a preferable result in which the tensile strength of the drawn steel wire having the diameter of 0.19 mm is 4,300 MPa or more.

The test numbers beyond the conditions stipulated by the present invention did not satisfy at least one of the performance described above. Hereinafter, the test numbers beyond the conditions stipulated by the present invention will be described.

In the test number 1, the C content was beyond the range of the present invention. In the test number 3, Mn+2Cr was beyond the range of the present invention. In the test number 5, the Cr content was beyond the range of the present invention. Therefore, in all of these test numbers, the tensile strength of the drawn steel wire was insufficient.

In the test number 6, Mn+2Cr was beyond the range of the present invention. In addition, martensite was generated in the wire rod. Consequently, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 8, the Cr content and Mn+2Cr were beyond the range of the present invention. In addition, the area fraction of pearlite did not satisfy the range of the present invention, and martensite was generated in the steel structure. Consequently, the drawn steel wire was broken at the time of dry wire-drawing and wet wire-drawing.

In the test number 10, the C content was beyond the range of the present invention. In addition, the area fraction of cementite did not satisfy the range of the present invention. Consequently, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 11, the Al content was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of wet wire-drawing.

In the test number 12, the Ti content was beyond the range of the present invention. In the test number 13, the N content was beyond the range of the present invention. In addition, in both the test numbers, the maximum grain size of TiN did not satisfy the range of the present invention. Consequently, the drawn steel wire was broken at the time of wet wire-drawing.

In the test number 14, the O content was beyond the range of the present invention, and the evacuation time was 10 minutes. Therefore, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 15, the average cross-sectional area of the ingot was not preferable. Consequently, the maximum grain size of TiN and the ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 were beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 16, a mold formed of a material of silica was used at the time of casting. Consequently, the maximum grain size of TiN and the ratio of the maximum value to the average value of Mn+2Cr in the central portion 11 were beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 17, the average cross-sectional area of the ingot was not preferable. In the test number 18, the volume fraction with cutting both ends of the ingot was 5%. Consequently, in both the test numbers, the maximum grain size of TiN was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of wet wire-drawing.

In the test number 22, Mn+2Cr was beyond the range of the present invention, and martensite was generated in the steel structure. Consequently, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 24, the test number 25, the test number 27, and the test number 43, the heat treatment conditions of the ingot were not preferable. Consequently, the ratio of the maximum value to the average value of Mn+2Cr in the central portion 11 and the ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 were beyond the range of the present invention. Therefore, in all of these test numbers, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 26, the heat treatment conditions of the ingot were not preferable. Consequently, the ratio of the maximum value to the average value of Mn+2Cr in the central portion 11 was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 28, the finish rolling temperature was not preferable. Consequently, the area fraction of cementite was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of dry wire-drawing and at the time of wet wire-drawing.

In the test number 29, the average cooling rate to 700° C. was not preferable. In the test number 30, the test number 39, and the test number 40, the average cooling rate of 700° C. to 610° C. was not preferable. Consequently, the area fraction of cementite was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of dry wire-drawing.

In the test number 33, the average cooling rate of 700° C. to 610° C. was not preferable. In the test number 34, the heating temperature of the steel billet was not preferable, and the finish rolling temperature was not preferable. Consequently, in both the test numbers, the area fraction of pearlite was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of dry wire-drawing.

In the test number 45, the heat treatment conditions of the steel billet before hot rolling were not preferable. Consequently, the ratio of the maximum value to the minimum value of Mn+2Cr in the outer circumferential portion 12 was beyond the range of the present invention. Therefore, the drawn steel wire was broken at the time of wet wire-drawing.

Hereinabove, a preferable embodiment and Examples of the present invention have been described. However, the embodiment and Examples are merely examples within the scope of the gist of the present invention, and addition, omission, replacement, and other changes of the configuration can be made within a range not departing from the gist of the present invention. That is, the present invention is not limited to the foregoing description and is limited to only the disclosed Claims. It is natural that the present invention can be suitably changed within the scope thereof.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

1 wire rod

11 central portion

12 outer circumferential portion

13, 14 linear analysis region 

1. A wire rod comprising, as a chemical composition, by mass %: C: 0.90% to 1.20%; Si: 0.10% to 1.00%; Mn: 0.20% to 0.80%; Cr: 0.10% to 0.40%; Al: 0% to 0.002%; Ti: 0% to 0.002%; N: 0% to 0.0050%; P: 0% to 0.020%; S: 0% to 0.010%; O: 0% to 0.0040%; Mo: 0% to 0.20%; B: 0% to 0.0030%; and a remainder including Fe and impurities, wherein an average value of % Mn+2x % Cr over an entirety of the wire rod is 0.50% to 1.00%; wherein 90% or more of a metallographic structure is pearlite by area fraction, and a remainder of the metallographic structure includes one or more of ferrite, cementite, and bainite, and the area fraction of the cementite is less than 3%; wherein a maximum grain size of TiN is less than 15 μm; wherein a maximum value of % Mn+2x % Cr measured on a cutting surface having a right angle to a longitudinal direction of the wire rod in a region where both a S content and an O content are less than 1% in a central portion, which is a region from a central axis of the wire rod to 1/10 of a diameter of the wire rod, is 2.0 times or less than the average value of % Mn+2x % Cr over the entirety of the wire rod, a ratio of % Mn+2x % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod of the maximum value to a minimum value in a region where both a S content and an O content are less than 1% in an outer circumferential portion, which is a region from an outer edge of the central portion to a depth of 0.1 mm from a surface of the wire rod, is 2.0 or less, and % Mn and % Cr indicate the amounts of Mn and Cr by mass %.
 2. The wire rod according to claim 1 comprising, as the chemical composition, by mass %: one or more of Mo: 0.02% to 0.20%; and B: 0.0003% to 0.0030%. 